Radio rebroadcasting system



March 10, w42-l E. H. ARMSTRONG RADIO REBROADGASTING SYSTEM s Sheets-sheet 1 Filed Jan. 12, 1940 March l0, 1942. E. H. ARMSTRONG RADIO REBROADCASTING SYSTEM Filed Jan. l2, 1940 3 Sheets-Sheet 2 March 1o, 1942.

E. H. ARMSTRONG l RADIO REBROADCAST'ING SYSTEM Filed Jan. l2, 1940 3 Sheets-Sheet 3 Patented Mar. 10, 1942 UNITED STATES PATENT OFFICE l RADIO REBROADCASTING SYSTEM Edwin H. Armstrong, New Y'ork, N. 1;'. Application January 12, 1940, Serial No. 313,498

8 Claims. The object of this` invention is to' provide a `relay system for frequency modulated signals,

and particularly for a system in which the relayed or retransmitted signal is radiated at a considerably different frequency than the initial frequency.

The particular problem which is to be solved will appear from the following explanation. At the present time there are in operation in the broadcast service a number of frequency modulated transmitters using the wide band fre'- quency modulated system described in '.ny U. S. Patent #1,941,069 which are transmitting on frequencies in the vicinity of 40 megacycles. It is desirable to locate these transmitters in places where the antenna, may be raised as high as possible above the surrounding country and this frequently results in their location onV mountain tops some considerable distance from the point of origin of the program to be transmitted. On account of the practical diiculty and costvof Wire lines, radio relaying has been used from the studio to the broadcast station making use of much higher frequencies than that transmitted in the broadcast service. The frequency of transmission has been usually in excess of 100 megacycles.

There are two types of relay systems. In one the signal from the relay transmitter is detected and converted into its original form and this-current is used to modulate the broadcastqtransmitter, In the second type thereceived radio frequency current is heterodyned to the frequency to be rebroadcast, amplied up and transmitted without ever being converted into the audio or other signaling current. In each type of relaying certain difficulties are encountered. In the rst type, reception is carried out with an ordinary frequency modulated receiver of sufhclent selectivity to distinguish between the received and retransmitted signals without overloading the initial stages of amplification. The signals are detected in the usual manner and applied to the modulation of the transmitter in the usual way. This method suffers from the fact that in the process of detection and remodulation there is a certain amount of distortion which, while it is hardly detectable in a single relay when all elements of the system are in proper adjustment, requires a certain amount of attention to maintain in that condition. In the second method in which the received signal is heterodyned directly to the new frequency without the process of detection and modulation this diiiiculty is avoided but a new one occurs.

In the first method it will be observed that the frequency of 'each transmitter is determined solely lby means located directly at each station and that the frequency of the retransmitted wave is independent of any changes in the frequency of the relay transmitter. In the second method, the frequency of the second station depends on the constancy of-the frequencyv of the first station. Hence where the frequency of the relay transmitter is high any drift in its frequency is accentuated in the beat frequency created at the second stationl by `tiie interaction between it and the locally lgenerated frequency. It is the purpose of this invention to -obtain the advantages of both methods without any of the disadvantages by arranging that the frequency of the second transmitter shall be substantially independent of the frequency of the first or relay transmitter, and responsive only to the frequency modulations therein, these frequency modulations being imparted tothe Wave transmitted from the second station directly, and hence without the intervention of the processes of. detection and modulation.

Referring now to the gureswhich form a part of this specification:

Figures I and II illustrate the general principle of the method and Figures III and IV illustrate more detailed means of carrying it out in practical form. y

The general theory of operation may be understood by reference to the arrangements of Figs. I and II. Fig. I illustrates in a purely schematic way a frequency modulated transmitter in which I represents the antenna, 2 a power amplifier for feeding the antenna and 3 a modulator for modulating the frequencyy to be radi" ated in accordance with the signaling currents to be transmitted. In accordance with the present invention' the modulation is supplied with two sources of modulating current; one the ordinary speech or musical currents of audible frequency,v and two, a superaudible current of constant amplitude and frequency which will be assumed to be of the order of 100 k. c. per second. For the purposes of this explanation it will be assumed that the transmitted wave has a frequency of 130,000,000 cycles and that the deviation or change in frequency from this point is 50,000 cycles above and below for the modulations which correspond to the speech frequencies. The purpose of the 100,000 cycle modulating current will appear hereinafter.

Referring now to the receiver and the second ytransmitter shown in Fig. II, 6 represents the 55 antenna connected through the usual tuned cirl rent of 1,000,000 cycles is passed through a filter 9, which has a band Width of something in excess of 200,000 cycles, amplified by the amplifier I as in ordinary superheterodyne practice and passed through a limiter II. In the output of this limiter are connected a wide band and a narow band filter, the wide band filter I2 passing a band which may be approximately 925-1075 k. c. and a narrow band filter I3 whose band width may be from 1075 to 1125 k. c. I4 represents an amplier for the output of the narrow band filter, I5 a converter whose heterodyning current is supplied from a crystal oscillator I6 having a frequency which will be assumed to be 1500 k. c. The output of the converter is passed through a filter I'I having a band pass of 2575 to 2625 k. c., and the output of this lter is supplied to a second converter I8. 'I'he converter I 8 is likewise supplied with a current from the output of .the band filter I2. The ou-tput of the converter I8 is passed through a filter I9 which has a band width of 1525 to 1675 k, c. Referring now to the transmitter part of this figure, 20 represents a crystal or other fixed frequency oscillator generating some submultiple of the frequency 41,400 k. c. 2I represents a multiplier or a series of them for raising the frequency of the oscillator to 41,400 k. c. 22 represents a converter for mixing the outputs of I9 and 2|. 23 represents a lter, 24 a power amplifier, and 25 the retransmitting antenna.

The method of operation is as follows: the transmitter when unmodulated radiates a single frequency of 130,000,000 cycles. When modulated fully in accordance with the assumed deviation of 50,000 cycles .with the speech frequency the band width occupied is 129,950,000 cycles to 130,050,000 cycles. Now assume -that the 100 k. c. modulation is applied. If the deviation is not made too great, two new frequencies only will appear at 129,900,000 cycles and 130,100,000 cycles, the other frequencies of the series being relatively insignificant. These frequencies are radiated continuously and at constant amplitude. The frequency band radiated in response to the speech or other signaling frequencies varies from a maximum width of 100,000 cycles down to zero in accordance with the strength of the signaling currents. Turning now to the receiver of Fig. II, the incoming 130,000,000 cycle current is heterodyned down to 1,000,000 cycles. When the ,transmitter is fully modulated by the signaling current and is also modulated at the auxiliary frequency of 100,000 cycles there will appear in the intermediate frequency circuit of the receiver the following currents: 900,000 cycles, 1,100,000' cycles and a band of frequencies running from 950,000 cycles to 1,050,000 cycles. All of these currents are passed through the limiter II and appear in its output circuit. where the frequency band representing the signaling frequency is selected by the broad band filter I2 and the upper frequency 1,100,000 cycles is selected by the narrow band filter I3. The 1,100,000 cycle current is then amplified and converted to 2,600,000 cycles by adding i to it a 1500 kilocycle current produced by the crystal oscillator I6. The filter I1 selects this frequency and supplies it to a converter I8 where it is combined with the output current from the broad band filter I2 which comprises the frequency band of the signaling currents. The difference frequency which varies between 1550 and 1650 k. c. is selected by the filter I9. It will be observed that by this process a frequency modulated current whose variations or modulations correspond in every respect to the modulations of the megacycle current has been produced yet whose midfrequency is independent of any changes or drift in frequency of the transmitted wave. It will also be observed that this is accomplished by transmitting an auxiliary frequency which bears a fixed frequency relation to ythe midfrequency of the modulated wave, deriving a heterodyning current from this auxiliary frequency and beating it against the frequency changes in the signaling current. Instead of beating the auxiliary frequency of 1,100,000 cycles directly against the signal modulated components it will be observed that the frequency of the auxiliary current is first raised to a higher value. In the present case a current produced by the oscillator I6 of 1,500,000 cycles is added to it to raise the frequency to 2,600,000 cycles. While in principle this is not necessary it has the effect of making the final output current of a frequency which is more manageable for the subsequent operations in the ltransmission and enables the circuits to be more effectively designed.

To carry out the process of transmission, it now only remains to impress the above described frequency modulations upon the second transmitter. This is accomplished by combining in a converter stage a locally generated fixed frequency current with the output of the filter I9 whose frequency differs from the frequency to be radiated by an amount equal to 1,600,000 cycles. Supposing the radiated frequency is to be 43,000,000 cycles it follows that a fixed frequency oscillator 20 generates some submultiple of 41,400,000 cycles which current is produced by the frequency multiplier 2|. The converter 22 then produces 'the sum and difference frequencies and, in the present case, the sum frequency is selected by the filter 23, amplified by the power amplifier 24 and transmitted to the antenna 25. As the 1,600,000 cycle current varies between the limits of 1,650,000 and 1,550,000 cycles these variations or modulations are imparted to the transmitd wave resulting in a plus and minus variation of 50,000 cycles about the midpoint of 43 megacycles.

It will be observed from the above description that the dependency of .the'frequency of the wave transmitted from the second station on the frequency of the wave transmitted from the first station has been entirely removed. The stability of the frequency of the wave transmitted from the second station depends in the main on the stability of the oscillator 20 plus the stability of two other oscillators, both of which are of very low frequency. Likewise at no point in the process are any of the undesirable features of detection and remodulation necessary so that the system has in effect no more distortion than the system which was described in the beginning of this specification in which the incoming signal was heterodyned directly to the frequency to be retransmitted.,

It will, of course, be obvious that while an example of the transmission of speech together with appropriate values of frequencies has been given that the system is capable of adaption to the transmission of any form of intelligence provided the values are properly selected. It will likewise be understood that various methods of deriving solely the frequency changes from the transmitted wave may be employed without departing from the spirit of the invention.

An alternative method of producing the result is illustrated in the arrangement shown in Fig. III, which illustrates a somewhat different form of transmission system. In this arrangement the auxiliary high frequency is transmitted separately and the frequency difference between it and the main frequency is determined in another way. Referring vnow to Fig. III, represents a master oscillator, 3|, 32 the carrier amplifier and balanced modulator of the usual phase shifting device, 34 an amplifier and 35, 36 a series of frequency multipliers. The principle employed in my co-pending application for U. S. patent, Serial No. 313,497, filed January 12, 1940, is employed in this system so that changes in the frequency of the oscillator are balanced out. 38 represents an amplifier and 39, 40 the frequency multipliers which form a source of heterodyning current for the converter 31 as will be hereinafter explained. The output.

of the second frequency multiplier path is supplied to a converter 4|. 43 represents an oscillator whose frequency is some submultiple of the frequency which it is desired to radiate and 42 a filter for selecting one of the frequencies resulting from the combination of the output of thel frequency multiplier 40 and the oscillator 43. 44 represents a series of frequency multipliers for raising the output current of the converter 31 to the frequency to be'radiated, 45 a power amplifier for this frequency and 46 the antenna for radiating it. This channel produces the main signaling current. The auxiliary frequency is produced by the other channel. Here 41 represents an amplier for the current produced by the oscillator 43, 48 a converter, 49 another oscillator whose purpose will be expla-ined later and 50 a crystal filter for selecting the desired frequency resulting from the .combination in the converter of the currents from 41 and 49. 5| represents an amplifier, 52 a series of frequency multipliers and 53 a power amplifier whose outputA is supplied to the antenna 54.

The principle of operation of the system will appear from the following explanation. Assume the same values of frequency for the main channel and the auxiliary channel, namely, 130,000

k. c. and 130,100 k. c. In accordance with a usual practice and as described in my co-pending application Serial No. 313,497, filed January 12, 1940, the initial frequency of 200 k. c. y is multiplied up by the frequency multipliers 35,

36 to 12.8 megacycles and supplied to the converter 31. This constitutes the modulated path. The unmodulated pathv through the amplifier 38 is likewise multiplied up through the multipliers 39, 40 to 12.8 megacycles, and supplied to the converter 4|. An4 oscillator 43, preferably crystal controlled, generates a frequency of 2708.3 k. c. which is likewise supplied to the converter 4| producing, among others, a new frequency of 10,091.7 k. c.v 'I'his frequency is selected out by the lter 4,2 and supplied to the converter `31 to produce, in combination with the 12,800 k. c. current a frequency of 2708.3 k. c. This frequency is then multiplied up by the multiplier 44, amplified by the power amplifier 45' and radiated by the antenna 46 at 130,000

k. c. 'I'he method so far is similar to that dequency changes inthe 200 k. c. oscillator are automatically compensated for by the method of deriving the heterodyning current for the converter 31 from the 200 k. c. crystal oscillator. It will be observed that the midfrequency of the radiated band is a multiple of the frequency of the oscillator 43, being exactly 48 times this frequency. It is now possible to secure the desired auxiliary frequency k. c. away from the midfrequency of themain channel by adding to the frequency of the oscillator 43, a frequency which is one forty-eighth part of 100 k. c. and multiplying the resultant frequency 48 fold. A current-of 2708.3 k. c. is,supplied to the converter 48 through the amplifier 41 and there combined with a current of 2116 cycles produced by the oscillator 49. The sum of these two frequencies results in a current of 2710.416-1- k. c. which is selected out from the other currents by the crystal filter 50, amplified by 5|, multiplied 48 fold by the frequency multipliers 52, and supplied to the antenna 54 through the power amplifierv53, at 130,100 k. c. In this manner a frequency exactly 100 k. c. from the midpoint of the main frequency can be .accurately obtained.

. While separate power amplifiers and separate antennas have been shown, it will be obvious that they may be used in common although the use of separate elements has certain advantages. Various other combinations may be used without departing from the spirit of the invention. The transmission may be received by the arrangement of Fig. II provided the power of the auxiliary frequency is kept below the level of the main transmission. However, a preferable arrangement for reception is shown in Fig. IV.

This arrangement, while generally like that of the receiver of Fig. II, differs from it in an important aspect. This consists in separating the main channel currents from the auxiliary cur- `rent by means of the broad and narrow filters 66 and 65 respectively, before limiting and making use of separate limiters 10 and 68. Other- Wise the action of the receiver is the same as in Fig. II the resulting final output current at 19 is supplied to the transmitter as in Fig. II. It will, of course, be understood that while a single intermediate frequency receiver `has been shown for the sake of simplicity that a double intermediate frequency one may be used. Such a receiver is in fact desirable when frequencies as high as megacycles are being dealt with and it will likewise be understood that the automa-tic tuning devices which are now well known in the transmitter current of 41,400 k. c. to produce the 'are supplied to a converter 80 which is also excited by av current derived from an oscillator 82 of 10.35 megacycles. One of the resulting output currents has a frequency of 11.95 megacycles which may be readily separatedl from the 10.35 megacycle current. This is accomplished -by raising the frequency in two stages that the filtering problem has practically disappeared.

I have described what I believe to be the best embodiments of my invention. I do not wish, however, to be conned to the embodiments shown, but what I desire to cover by Letters Patent is set forth in the appended claims.

I claim:

1. 'I'he method of signal transmission which consists in frequency modulating a carrier wave with a band of signal frequencies, transmitting the modulated carrier wave, transmitting a Wave having a constant frequency, receiving the transmitted waves, heterodyning the received waves down to a low intermediate frequency by a heterodyning current of constant frequency, selecting from the low intermediate frequency a band of frequencies corresponding to the signal frequencies and a derived frequency differing from the median intermediate frequency by the difference in frequencies between the carrier and the wave of constant frequency, beating said derived frequency with the selected band of frequencies to produce a resultant band of frequencies corresponding to the signal frequencies, and causing the frequency of a. second carrier wave of different frequency than the first named carrier wave to vary in accordance with the frequencies of said resultant band.

2. The method of signal transmission which consists in frequency modulating a carrier wave with a band of signal frequencies, transmitting the modulated carrier wave, transmitting a wave having a constant frequency differing from the frequency of the carrier wave by a frequency above audibility, receiving the transmitted waves, heterodyning the received waves down to a low intermediate frequency by a heterodyning current of constant frequency, selecting in parallel paths from the low intermediate frequency a derived frequency differing from the median intermediate frequency by the difference in frequencies between the carrier and the wave of constant frequency, and a band of frequencies corresponding to the signal frequencies; increasing said derived frequency to provide a higher frequency, beating said higher frequency with the selected band of frequencies to produce a band of frequencies corresponding to the signal frequencies, and causing the frequency of a second carrier wave of different frequency than said first-named carrier wave to vary in accordance with the frequencies of said band.

3. In a radio transmission system, the combination of means for transmitting a band of frequency modulated signals on a carrier wave, means for transmitting a wave having a constant frequency, a first converting device, means for receiving the transmitted waves and impressing them on said converting device, a local oscillator connected to said converting device and arranged to generate a frequency differing from the frequency of the carrier wave by a low intermediate frequency, a pair of parallel paths connected to the output of said converting device.

the first of said paths being arranged to block currents in the middle of the low intermediate frequency band and to pass currents diering in frequency from the mid-intermediate frequency by an amount equal to the difference in frequencies between the carrier wave and the wave of constant frequency, and the second of said paths being arranged to pass a low intermediate frequency including the frequency modulations of the signal, a second converting device connected to said parallel paths to provide in the output thereof a resultant frequency including the frequency modulations of the signal, means for increasing the resultant frequency to the frequency to be transmitted, and means for transmitting currents of the resulting increased frequency.

4. The combination as set forth in claim 3 in which a local oscillator generating a constant frequency is connected to one of said parallel paths to increasel the frequency of the currents l therein.

5. The combination as set forth in claim 3 in which an amplifying device adapted to amplify a wide band of intermediate frequency currents is connected between the parallel paths and the first converting device.

6. In a frequency modulation signal transmission system, in combination. a first generating device arranged to generate radio frequency oscillations, a rst converting device, a circuit connecting said converting device and said generating device and comprising means for multiplying the frequency generated by the generating device, means for frequency modulating the currents in said circuit in accordance with the signals, a second converting device having its output connected to the input of said first converting device, a circuit connecting the input of the second converting device and said generating device and comprising means for multiplying the frequency generated by the generating device, means connected to the output of said first converting device for transmitting frequency modulated signaling currents, a third converting device, a second generating device arranged to generate radio frequency oscillations and connected to the inputs of the second and third converting devices, a third generating device connected to the input of said third converting device and arranged to generate a frequency which is a submultiple of the difference between the frequency of the frequency modulated currents and the frequency of the constant frequency currents and means connected to the output of the third converting device for transmitting the currents of constant frequency.

7. In a system for rebroadcasting frequency modulated carrier waves, the -method of preventing changes in the` frequency of vthe transmittedwave due to variations in the frequency of the received wave which consists in frequency modulating a carrier wave with a band of signal frequencies, transmitting the modulatedl carrier wave and also a second wave whose frequency differs from the frequency of the carrier w'ave by a substantially constant amount, receiving the transmitted waves, heterodyning the received waves down to a low intermediate frequency. selecting from the low intermediate frequency the band of signal frequencies andy a derived frequency differing from the median intermediate frequency by the aforesaid constant amount. beating said derived frequency with the selected band of frequencies to produce a second band of frequencies corresponding to the signal frequencies, increasing the frequency of said second band to the frequency to be transmitted and transmitting currents of the resulting frequency.

8. A system for rebroadca'sting frequency modulated carrier waves comprising, in combination,

means for generating a carrier wave, means for frequency modulating the carrier Wave with a band of signal frequencies, means for transmitting the carrier wave and also a second wave whose frequency differs from the frequency of the carrier Wave by a substantially constant amount, means for receiving the transmitted waves, means for heterodyning the received Waves down the selected band of frequencies to produce a' second band of frequencies corresponding to the signal frequencies, means for increasing the frequency of said second band to the frequency to ybe transmitted and means for transmitting currents of the resulting frequency.

EDWINl H. ARMSTRONG. 

